HVAC
HVAC is an acronym for Heating, Ventilation & Air Conditioning. Earth sheltered homes are super efficient and, when combined with passive solar heating, heating and air conditioning is less of an issue than with a standard home, but it is still an important issue that needs to be done right.
This page is about the technical aspects of HVAC, for sourcing (costs), try this other page.
Why do I need a heating system?
Thermal comfort is one of the most important things that make a home a home. Some people think luxury means fancy marble or gold plated objects, but I think it is more about feeling comfortable. Would you rather have a gold plated shower head or one that always put out water at the perfect temperature and pressure? A thermally moderated shower is pretty expensive actually (its weight in gold), and so are the other systems needed for making our homes thermally “comfortable”. And yet, when it comes time to build a home, most people put more effort into choosing the paint colors than the heating system.
As I mentioned on the earth sheltering and passive solar pages, these two concepts go so naturally together that it is assumed that you will try to heat your earth sheltered home with passive solar energy. There are varying degrees of success with this approach. Some claim that their homes perform very well on only solar heat and internal gains such as the waste heat from their refrigerator and party guests.
Most heat load calculators use the rough number of 400 BTUs/hr of free “waste heat” from each person in your home.
Other earth sheltered home owners report using less than half a cord of wood all winter. Many have installed heating systems and then never turned them on, or report that they only used them for the first few years before the earth temperatures around the home stabilized. One author (I read too many books to recall which) referred to the furnace he installed as an expensive Cadillac that he only drove a few days a year. All earth sheltered enthusiasts could at least agree that an earth sheltered home doesn’t need the same size heating system that the equivalent size above ground home requires to remain comfortable (and livable) in the winter.
If you are building in an area without much government oversight and paying for it all yourself, you can do what you like. But if you want a mortgage, you should expect that your banker is probably not an earth sheltered enthusiast and will probably want to be sure that your home has a “reasonably-sized” thermostat-controlled heating system. They are investing in your house, so they have a say. If you ever want to sell your house, potential buyers may also prefer that such a “backup” system is in place.
The building inspector will likely be more neutral. The mechanical code talks about how the natural gas lines (if applicable) should be hooked up, and how close to the furnace your electrical outlet should be, etc. but it doesn’t actually say you need a furnace. If you do have one installed, it will need to be done according to code, but if it is your own residence, you are allowed to do it yourself.
Do you need a license to do mechanical work?
A mechanical contractor’s license with the appropriate classification is required for the performance of installing, altering, or servicing the following: (1) Hydronic heating and cooling and process piping, (2) HVAC equipment, (3) Ductwork, (4) Refrigeration, (5) Limited Heating service, (6) Unlimited Heating Service, (7) Limited Refrigeration and air conditioning service (8) Unlimited refrigeration and air conditioning service, (9) Fire Suppression, (10) Specialty. However, an owner of a single family dwelling who will occupy or occupies the residence may perform the work without a license. ~mechanical permit information
There is also the chance that your home may not perform as well as you expect, or perhaps, next winter will be much colder than your calculations anticipated. Adding a heating system after the house is built is much harder than doing it during the design phase.
So, assuming this is at least worth some investigation, this page talks about heating systems, and more specifically, “backup” heating systems for passive-solar earth-sheltered homes.
Heating Systems
There are a wide variety of heating systems on the market that cover a wide range of cost and efficiency. Lets separate the discussion in two halves. Heat generation and heat delivery. Heat retention (insulation) is also important.
Rough notes for the heat deliver section
(under construction)
I knew that Geothermal was expensive. From the buried heat exchange tubes to the unit that goes in your house, it is probably the most expensive heating system, even after the Government gives you a 30% rebate. But after it is installed, it is long lasting and 4 to 5 times as efficient as any other system. I wanted hydronic heating, rather than forced air. Hydronic is more efficient and more uniformly distributes the heat in a way that feels more comfortable. Hydronic heating is quiet and you don’t feel drafts or blow dust around your house. In a passive solar design, hydronic can potentially be used to store solar heat in the fluid and move it around to less solar spaces.
Hyrdronic is more affordable during new construction than trying to retrofit for it later. Functionally, it goes well with my concrete floors.
There are some down sides. Without the furnace filter, dust simply settles to the floor and needs to be swept up. Randiant functions by first heating up the slab, which then slowly radiates out to the living space. The amount of mass involved adds a lot of inertia to the system, so it responds slowly to change. Adding carpets or rugs or even hard wood floors increases the resistance between the heated mass and the living space, further slowing the response time. Adding carpet could also increase the temperature of the slab, which can reduce the efficiency of the heat exchange with the hydronic fluid (heated water). This is more pronounced in warmer areas where the hydronic temperature is set to 85F rather than in northern areas where it is typically set to 160F.